As known in the art of combustion engine diagnostics, the torque on the crankshaft of an IC engine increases and decreases as the gas, or volume is in the cylinder compresses and expands during cranking of the engine. Early methods of analyzing the compression of each cylinder of an engine included analysis of the engine starter current magnitude during cranking since the required torque, and therefore starter current, increases on each compression stroke of the cylinder's piston. By comparison of the relative magnitudes of change in starter current associated with the compression stroke of each cylinder, the relative compression among all the cylinders of the engine could then be determined. One such apparatus for measuring the relative compression in an IC engine is disclosed in U.S. Pat. No. 4,050,296 of common assignee herewith, to R. S. Benedict where the relative compression is measured electronically by measuring excursions in a selected engine parameter which varies as a consequence of the compression stroke of each cylinder. In the system of Benedict the engine parameter measured may be starter current, starter voltage, or instantaneous sub-cyclic engine speed, all of which are measured as a function of time and indexed to the engine firing cycle through the use of a cylinder identification (CID) signal indicating a particular point, such as a particular cylinder piston position or the opening or closing of an exhaust valve, all of which occurs once within an engine revolution. This indexing of the measured parameter to a crankshaft event allows identification of each cylinder since the firing order and therefore the cyclic displacement of each cylinder piston from the crankshaft index is known.
In asymmetric IC engines where the crankshaft angle displacement between top dead center (TDC) of the engine cylinders is asymmetrical, i.e. the crankshaft angular value between TDC of a common bank of cylinders (left or right bank) is on even degrees submultiple of one revolution (360.degree. divided by the number of cylinders in the bank) whereas the crankshaft angle value between TDC of adjacent firing cylinders as defined by the engine firing order are asymmetrical as determined by the mechanical angular displacement of the two cylinder banks, or the inter-bank angle. As a consequence the crankshaft angle interval between TDC of adjacent firing cylinders is unequal. In the symmetrical firing IC engine the measurement of the engine parameter (starter current, etc.) as disclosed by Benedict is measured for equal increments on each cylinder (equal compression stroke increments), such that the relative contribution parameter is readily determined. In the asymmetric engine, however, the variation in compression stroke intervals due to the asymmetric spacing of adjacent cylinder TDC results in unequal excursions of the selected engine parameter for the adjacent cylinders. This is due to the fact that the most closely spaced cylinders overlap on their compression strokes such as to mask the change in selected parameter for the second one of the two cylinders in the firing order. This prohibits obtaining a relative compression value indication when half of the cylinders contribute a substantially smaller change in the selected parameter than the remaining half. While it is true that the relative compression values may be given for each bank which itself may provide some useful indicator of overall compression, this does not provide the compression information required for the advance diagnostics.